1. Field of the Invention
The present invention relates to optical components in silicon oxide and/or other mixed metallic oxides having dimensional precision which has surface roughness tolerance and profilometric accuracy within the specifications described for visible and ultraviolet spectrum ranges.
The above manufactured articles have xe2x80x9cfinalxe2x80x9d or xe2x80x9calmost finalxe2x80x9d dimensions as they are obtained by the isotropic dimensional reduction (miniaturization) of amorphous monolithic materials, called aerogels, in turn prepared by means of cold moulding techniques based on sol-gel processes.
The process for the preparation of the above objects involves the accurate geometrical definition of the aerogel by:
the cold filling of a suitable mould with a liquid colloidal dispersion, called sol, formed from specific chemical precursors;
the polycondensation of the sol to obtain the respective gels therefrom (gelation);
the supercritical drying of the gels until aerogels are obtained with dimensions corresponding to the mould used;
the isotropic reduction (miniaturization) of the amorphous monolithic aerogels thus obtained, consisting of silicon oxide alone or in the presence of one or more oxides of elements belonging to groups III to VI of the Periodical Table and exceptionally also other groups.
2. Description of the Background
It is known that optical materials, and in particular transparent optical materials such as silica or molten quartz and optical glass, owing to their hardness and fragility, are difficult to process as the direct hot moulding of these optical components and devices is generally not possible for reasons of product quality.
The traditional method for producing these optical elements is based on the reduction of an adequate preform to the end product by means of slow, precise grinding operations.
Whereas some of these operations, such as reduction with both a flat and spherical surface, can be automized, others, such as aspherical finishing, require complicated manual processes.
This operational difficulty results in a limited process flexibility on an industrial scale and unreasonably high costs to obtain quality products such as optical components and devices based on the above aspherical optical system.
Owing to these technological limitations the optical industry has tried to solve the problem in various ways.
One of these is the moulding at high pressure and temperature of aspherical lenses and other optical components, directly from appropriate preforms of the optical material desired; with this method, which requires extremely sophisticated equipment such as a hot hydrostatic press, high quality products are obtained but also at a high cost and the process consequently necessitates very substantial investments.
One way of reducing the costs is by the use of organic optical materials, in particular plastics.
These materials can be melted and moulded with much more economical processes and can also be very easily processed with machine tools.
Unfortunately the dimensional precision of the optical products obtained generally by melting, is negatively influenced both by the insufficiently controllable shrinking of the material during the cooling operation and by the change of liquid-solid phase which causes a dimensional distorsion and deterioration of the optical quality of the manufactured article.
Also with the use of mechanical processing with machine tools, the optical products obtained from plastic materials do not have an acceptable quality as the material cannot be accurately processes owing to the fact that it is too soft.
In addition, the products which can be obtained with the above plastic materials, by hot moulding or mechanical processing, suffer from limited chemical and dimensional stability and do not reach the durability standards established for inorganic optical materials.
It is also known that optical components with definite dimensions can be obtained by suitably treating a gel deriving from the hydrolysis of a silicon alkoxide.
For example, U.S. Pat. No. 4,680,049 describes a method for the preparation of optical glass based on silicon oxide which involves an initial hydrolysis of a silicon alkoxide, the drying of the above gel and a final thermal syntherization treatment until an optical glass with definite dimensions is obtained.
These xe2x80x9cfinalxe2x80x9d optical products however have a very significant deviation with respect to the profile of the aerogel, as is amply illustrated in FIG. 1.
The two diagrams shown in the above figure represent the configuration of the upper surface of the aerogel (diagram A) and the corresponding surface of the densified product (diagram B) respectively.
In the mould in which the gel is prepared the corresponding surface is rigorously flat: it can be seen how the flat surface of the mould passes to a convex surface in the aerogel to end up as a concave surface in the densified product.
The distorsion of the manufactured article is herein quantified as follows:             distorsion      ⁢              xe2x80x83            ⁢      from      ⁢              xe2x80x83            ⁢      mould      ⁢              xe2x80x83            ⁢      to      ⁢              xe2x80x83            ⁢      aerogel        =                                        20            ⁢                          xe2x80x83                        ⁢            μm                                3000            ⁢                          xe2x80x83                        ⁢            μm                          xc3x97        100            =              0.67        ⁢        %                        distorsion      ⁢              xe2x80x83            ⁢      from      ⁢              xe2x80x83            ⁢      aerogel      ⁢              xe2x80x83            ⁢      to      ⁢              xe2x80x83            ⁢      glass        =                                        40            ⁢                          xe2x80x83                        ⁢            μm                                2000            ⁢                          xe2x80x83                        ⁢            μm                          xc3x97        100            =              2        ⁢        %            
This process, which herein is simply indicated as xe2x80x9ccompensated distorsion processxe2x80x9d, is severely limited in its industrial applications as there are difficulties in programming specific geometries of the product.
In fact, as there is no biunivocal, continuous correspondence between the geometry of the mould and that of the product, there is also no rational control of the final dimensions of the product itself.
Another attempt at developing the processing technology of optical materials has been made using machine tools with a very high precision numerical control, having a diamond point so as to be also able to process hard materials such as quartz and optical glass and with movement on air bearings to minimize the vibrations of the tool point.
These machines have been successfully developed in the last ten years and reach precision in the profile control of about a tenth of a micrometer and, under favourable conditions, even higher precision in the control of the surface roughness; they are consequently capable of finishing an item with so-called xe2x80x9copticalxe2x80x9d precision, which means a precision which is suitable for optics limited within the infrared spectrum range.
On the other hand, the above machines are still not adequate for applications in visible and ultraviolet spectrum ranges owing to the more severe specifications of surface roughness and profilometric accuracy required by optical laws within these spectrum ranges.
In addition, this high precision processing, which although economically convenient in special applications such as mirror finishing by laser in copper, aluminium or other materials typically used in infrared, is not generally economical for obtaining transparent optical components based on silica or inorganic glass, for numerous reasons including the hardness and fragility of the materials.
It is known in fact that these machines can be well used in the processing of typical materials for applications in infrared; this is due to their processability characteristics which are much higher than optical glass.
This creates great difficulties in the spectrum ranges, where glass is the prevalent material for which the technology of the single rotating diamond point (S.P.T.D.M.) cannot be used because of its fragility.
As described in Italian patent application MI-92A02038 filed by the Applicant, these high precision machine tools are used on intermediates to obtain perfectly and completely isotropic optical components and devices in xe2x80x9cfinalxe2x80x9d or xe2x80x9calmost finalxe2x80x9d dimensions; the above intermediates, as they have the property of isotropically shrinking, are monolithic aerogels ideally amorphous of silica and/or other metallic oxides produced according to the technolgy described in U.S. Pat. No. 5,297,814.
The Applicant has now found that gels prepared with the technology of U.S. Pat. No. 5,207,814, in suitable moulds, in accordance with what is described in Italian patent application MI92A02038 which can be referred to for any possible point of interest, can be linearly miniaturized into densified products which maintain the proportions of the mould with a precision greater than one part out of 10,000.